Various methods have been proposed for the continuous processing of granular solids. For example, the use of fluidized beds and moving packed beds are well known in the art. Fluidized beds are known to be very efficient in processes where the rapid mixing of solids is desirable. Fully fluidized beds (except when very shallow) are only able to handle particles of relatively uniform size, and as such are unsuitable for handling divided solids containing a broad range of particle sizes. The amount of material that can be handled per unit area of reactor cross-section in a fully fluidized system will be limited by the maximum operable bed depth.
A moving packed bed may be capable of handling a larger volume of material per unit area of reactor cross-section than a fluidized bed. However, a packed bed is not designed to mix solids should this be desired. In addition, in processes requiring the introduction of a gas into the bed, as for example a stripping gas or a reactant gas, non-uniform flow or bridging of the solids can result if the particle size is not uniform.
U.S. Pat. No. 4,199,432 discloses a process for retorting non-uniform particles of oil shale and other hydrocarbon containing solids using "a staged turbulent bed", a reactor design which achieves rapid local mixing of solids, a high solids throughput, and avoids the slugging problems associated with non-uniform particles. The text of this patent is herein incorporated by reference. In the staged turbulent bed, as described in the subject patent, raw shale particles and hot burned shale particles are introduced into an upper portion of a vertically elongated reactor vessel and pass downwardly therethrough. Heat transfer from the hot burned shale to the raw shale provides the heat for retorting.
The maximum particle size for the raw shale and previously retorted shale particles in such a process is normally maintained at or below 21/2 mesh, Tyler Standard Sieve size. Particle sizes in this range are easily produced by conventional means such as combinations of cage mills, jaw crushers, or gyratory crushers. Raw shale crushing operations may be conducted to meet a maximum particle size specification, but little or no control can be effected over the quantity of smaller particle sizes.
The temperature of the burned shale heat carrier introduced to the reactor is normally in the range of 1100.degree. F.-1500.degree. F. A correspondingly appropriate operating ratio of heat carrier to raw shale is then used to achieve the desired temperature in the reactor. The raw shale is introduced at ambient temperature or, if desired, preheated to reduce the heat requirement from the recycled heat carrier. The temperature at the top of the reactor is normally maintained within the broad range, 850.degree. F. to 1000.degree. F., and is preferably maintained in the range of 900.degree. F. to 950.degree. F.
The weight ratio of burned shale heat carrier to fresh shale may be varied from approximately 1.5:1 to 8:1 with a preferred weight ratio in the range of 2:1 to 3:1. It has been observed that some loss in product yield occurs at the higher weight ratios of burned shale to fresh shale and it is believed that the cause for such loss is due to increased adsorption of the hydrocarbonaceous product vapors by the larger quantities of burned shale. Furthermore, attrition of the shale, which is a natural consequence of retorting and combustion of the shale, occurs to such an extent that there is a maximum recycle ratio which can be achieved using burned shale alone. If it is desired to operate at higher weight ratios of heat carrier to fresh shale, alternative attrition resistant carriers, such as sand, must be provided as part or all of the heat carrier.
The mass flow rate of fresh shale through the reactor is normally maintained between 5,000 kg/hr-m.sup.2 and 30,000 kg/hr-m.sup.2, and preferably between 10,000 kg/hr-m.sup.2 and 20,000 kg/hr-m.sup. 2. Thus, in accordance with the broader recycle heat carrier weight ratios stated above, the total solids mass rate will range from approximately 12,500 kg/hr-m.sup.2 to 270,000 kg/hr-m.sup.2, preferably in the range 25,000-176,000 kg/hr-m.sup.2, and more preferably 30,000-78,000 kg/hr-m.sup.2.
A stripping gas is introduced into a lower portion of the reactor and passes upwardly through the vessel in countercurrent flow to the downwardly moving solids. The flow rate of the stripping gas is normally maintained so as to produce a superficial gas velocity at the bottom of the vessel in the range of approximately 30 cm/second to 150 cm/second, with a preferred superficial velocity in the range of 30 cm/second to 90 cm/second. The stripping gas may be comprised of steam, recycle product gas, hydrogen, an inert gas or any combination thereof. It is particularly important, however, that the stripping gas selected be essentially free of molecular oxygen to prevent combustion of products within the retort.
The stripping gas will fluidize those particles of the raw shale and heat carrier having a minimum fluidization velocity less than the superficial velocity of the stripping gas. Those particles having a fluidization velocity greater than the superficial gas velocity will pass downwardly through the retort, generally at a faster rate than the fluidized particles.
An essential feature of the staged turbulent bed retorting system lies in limiting the maximum bubble size and the gross vertical backmixing of the downwardly moving shale and heat carrier so as to produce stable, substantially plug flow conditions throughout the reactor volume. True plug flow, wherein there is little or no vertical backmixing of solids, allows higher conversion levels of kerogen to vaporized hydrocarbonaceous material than can be obtained, for example, in a fluidized bed retort of equivalent volume (where there is gross top to bottom mixing). Maintaining substantially plug flow conditions by limiting top to bottom mixing of solids, allows much greater control of the residence time of individual particles. Such control permits a substantial reduction in size of the retort zone required for a given oil production rate, since the chances for removing partially retorted solids with the retorted solids are reduced.
Gas bubbles in a fluidized bed coalesce to form larger bubbles as they rise. Oversized bubbles cause surging or pounding in the bed, leading to a significant loss of efficiency in contacting and an upward spouting of large amounts of material at the top of the bed. The means provided for limiting backmixing also reduces the coalescence of bubbles, thereby allowing the size of the disengaging zone to be somewhat reduced. The means for limiting backmixing and limiting the maximum bubble size can generally be described as baffles, barriers, dispersers or flow redistributors, and may, for example, include spaced horizontal perforated plates, bars, screens, packing, or other suitable internals.
Although gross vertical backmixing should be avoided, highly localized mixing is desirable in that it enhances the degree of contacting between the solids and between the solids and gas. Localized mixing necessarily introduces some backmixing and thus deviates from strictly plug-flow behavior. The degree of backmixing is dependent on many factors, but is primarily dependent upon the particular internals or baffles disposed within the retort.
Of great importance in the staged turbulent bed reactor is the interaction between the fluidized solids, the non-fluidized solids, and the internals employed for preventing backmixing. The fluidized solids generally proceed down the reactor as a moving fluidized columnar body. Without internals, a stable moving fluidized bed cannot be achieved with a solids mixture having a broad particle size distribution. The internals significantly affect the motion of the non-fluidized particles and thereby substantially increase the residence times of said particles. The average velocity of the falling non-fluidized particles, which determines said particles' residence time, is substantially decreased by momentum transfer to the fluidized particles and the retort vessel internals. The increased residence time permits the larger particles to be retorted in a single pass through the vessel. It has been discovered that with some internals, such as horizontally disposed perforated plates having a 49% free area and spaced throughout the vessel at one foot spacings, the residence time of the non-fluidized particles approaches the average particle residence time.
A reactor combining overall plug flow characteristics with intense local mixing provides the equivalent of a serial plurality of perfectly mixed stages. The term "perfectly mixed stage" as used herein refers to a vertical section of the retort wherein the degree of solids mixing is equivalent to that attained in a perfectly mixed volume having gross top-to-bottom mixing. The number of equivalent perfectly mixed stages actually attained depends upon many inter-related factors, such as bed height, gas velocity, particle size distribution and the type of internals selected to limit gross top-to-bottom mixing.
Excellent stripping of the volatizable hydrocarbonaceous product from the retorted solids is uniquely achieved with the staged turbulent bed reactor. With the staged flow characteristics, the "lean" stripping gas first contacts those particles having the least amount of adsorbed hydrocarbonaceous material, thus maximizing the driving force for mass transfer of the hydrocarbonaceous product into the fluidization gas stream.
As the hydrocarbon vapors evolved from the shale mix with the stripping gas, the gas velocity increases along the length of the reactor. The actual amount of increase will depend upon the grade of shale processed and the mass rate of fresh shale per unit cross-sectional area, but it may be minimized, if necessary, by proper initial design of the retort vessel itself. In this regard, the vessel may have an inverted frustoconical shape or may be constructed in sections of gradually increasing diameter.
The pressure at the top of the reactor is preferably maintained no higher than that which is required to accomodate downstream processing. The pressure in the bottom of the reactor will naturally vary with the height of the reactor and the chosen downstream equipment, but will normally be in the range of 2-4 atmospheres.
A product effluent stream comprised of hydrocarbonaceous material admixed with the stripping gas is removed from the upper portion of the retort by conventional means and passes to a separation zone. Since the product effluent stream will normally contain some entrained fines, it is preferred that said fines be separated from the remainder of the stream prior to further processing. This separation may be effected by any suitable means; conventional examples are cyclones, pebble beds and/or electrostatic precipitators.
The retorted shale along with the burned shale serving as heat carrier is removed from the lower portion of the retort by conventional means at the retort temperature. The retorted shale will normally have a carbonaceous residue content of approximately 2 to 4 weight percent and represents a valuable source of energy which may be used to advantage in the process.
The retorted shale and burned shale heat carrier are fed to a combustor which may be of any conventional design, but it is preferred that it be a dilute phase lift combustor. Air is injected into the lower portion of this combustor and the carbonaceous residue on the shale is partially burned as the shale is transported. This combustion heats the shale mixture to a temperature in the range of 1100.degree. F. to 1500.degree. F. and the hot shale and flue gas are removed from the upper portion of the combustor. A portion of said hot shale is recycled to the retort to provide heat for retorting. Preferably said recycled shale is classified to remove substantially all of the minus 200 mesh particles prior to introduction to the retort so as to minimize entrained fines carryover with the retort product vapors.
The present invention is directed to an improved staged turbulent bed reactor whose bed pressure drop closely approximates that observed in a fully fluidized bed. This has the advantage of reducing the energy required to pump the countercurrent flow of gas through the reactor.